Read-only optical record carriers (hereafter "record carriers") have become increasingly important as a storage medium for audio information data, video information data, and other data types because of the capacity to store large volumes of information for later reproduction. Demand for ever-larger storage capacities and smaller device size continues to rise, however, making it necessary to further increase the data recording density of the record carrier.
Conventional record carriers of this type are disc-shaped recording medium with a resin substrate to the surface of which are formed spiral or concentric data tracks of pits and lands. A reflective film of, for example, aluminum, is then formed by sputtering or another process on the data carrier surface of the substrate.
When data is reproduced from this record carrier, an optical beam emitted from a semiconductor laser is focused on the record carrier, and the laser beam is controlled to follow the data tracks of the record carrier by detecting the reflected beam. The recorded information is read by detecting the change in the reflected light quantity resulting from the pits and lands on the record carrier.
The phase difference methods described in U.S. Pat. No. 4,057,833 which was issued to Josephus et al. on Nov. 8, 1977 and U.S. Pat. No. 4,740,940 which was issued to Tanaka et al. on Apr. 26, 1988 have been used to detect the control signal for tracking control, i.e., a track error signal corresponding to the offset, or positional error, between the positions of the optical beam and the actual track on the record carrier.
This phase difference method uses a photo detector dividing the reflections from the record carrier into quadrants in the track length and width directions of the detection surface, and determines any track error based on the phase difference of the sum signals outputted by detectors at diagonally opposite positions.
A three-beam method has also been described in U.S. Pat. No. 3,876,842 issued to Gijsbertus on Apr. 8, 1975. This method emits three optical beams, the read beam and two complementary beams, to the record carrier, detects each reflected beam using a discrete photo detector, and detects the track error based on the light quantity, or the light density, difference of the complementary reflected beams.
The data recording density of the above record carrier is determined by the data track pitch and the data density in the track direction, i.e., the linear density of the recorded data. However, as the track pitch is decreased, crosstalk from adjacent tracks increases. When there is a strong correlation between data recorded on adjacent tracks, pseudo signals are generated in the track error signal, and tracking control is not stable. These phenomena under the phase difference method are described below with reference to FIGS. 16 and 17.
In FIG. 16, an example of a photo detector 104e used for receiving an optical beam that is preferably a laser beam reflected from the record carrier to detect the focus error signal, the tracking error signal, and the information signal is shown. The photo detector 104e is preferably constructed by four square cells C1, C2, C3, and C4, each cell is close to two other cells by two neighboring side edges, as shown in FIG. 16. Each of square cells C1, C2, C3, and C4 generates pilot signals Sc1, Sc2, Sc3, and Sc4, respectively, according to the area of laser spot focused thereon.
The tracking control of the laser beam is performed by utilizing these pilot signals Sc1, Sc2, Sc3, and Sc4 as follows. The pilot signals Sc1 and Sc4 produced from the cells C1 and C4 located in a diagonal position are summed to produce a first sub tracking signal ST1. Similarly, the pilot signals Sc2 and Sc3 produced from the cells C3 and C2 located in another diagonal position are summed to produce a second sub tracking signal ST2. According to the difference between two sub track signals ST1 and ST2, the laser beam Ls is tracked.
In FIG. 17, a plurality of pits P each having a simple spatial frequency are recorded on plural tracks Tr1, Tr2, and Tr3 along the center lines thereof are shown. A spot of the leaser beam Ls is positioned to scan the pits along the center line of track Tr2, and the laser beam reflected from thus scanned the track Tr2 is received by the photo detector 104e of FIG. 16.
The solid lines L1 and L2 indicate the first and second sub tracking signals ST1 and ST2, respectively, in an ideal condition where the scanned track Tr2 is free from any interference such as crosstalk from the neighboring tracks Tr1 and Tr3. The dotted lines L1d and L2D indicates the first and second sub tracking signals ST1 and ST2, respectively, in an actual condition where there are interferences between the tracks Tr1, Tr2, and Tr3.
Under the ideal conditions, the phases of both the sub tracking signals ST1 and ST2 are identical to those of pits P formed on the scanning track Tr2, as specifically indicated by solid lines L1 and L2. However, the pits existing on the neighboring tracks causes the pilot signals Sc1, Sc2, Sc3, and Sc4 reproduced from the scanned track Tr2 to have crosstalk with sub signals reproduced from the neighboring tracks Tr1 and Tr3.
This crosstalk in pilot signals Sc1, Sc2, Sc3, and Sc4 affects the phases of sub tracking signal ST1 and ST2 that are summation of a diagonal pair thereof. Specifically, one of sub tracking signal ST1 and ST2 is shifted forward, and the other is shifted backward with respect to a chronological order when the data recorded on the neighboring tracks have strong correlations therebetween. In this example, the first sub tracking signal ST1 is advanced by a period .DELTA.ta, and the second sub tracking signal ST2 is delayed by a period .DELTA.td, respectively.
Closer track pitch causes the greater crosstalk between neighboring tracks. Furthermore, when the data having the same or similar pattern are recorded on the neighboring tracks with the closer track pitch, the correlation between the neighboring tracks is great enough to increase the advanced period .DELTA.ta or delaying period .DELTA.td too much to perform the tracking the laser beam correctly according to the tracking signals ST1 and ST2.
Therefore, when the pits on one neighboring track, for example, Tr1 are located on advanced position to those on the currently scanning track Tr2, the correlation between these tracks Tr1 and Tr2 acts the signal reproduced from the current track Tr2 to advance. On the other hand, when the pits on the other neighboring track Tr3 are located on delayed position to those on the currently scanning track Tr2, the correlation between these tracks Tr2 and T3 acts the reproduced signal to delay.
In other words, when pits having a simple spatial frequency are recorded across plural tracks, the signal correlation between the signal from the track to which the optical beam is positioned is strong; and the signals from the adjacent tracks are also extremely strong. Crosstalk from these adjacent tracks thus disrupts the track error signal, resulting in unstable tracking control.
When digital images are recorded to such a record carrier, still images may obviously also be recorded. Though it is not a problem with moving images, signals with a strong correlation may be recorded across plural tracks in still image recordings, and tracking control becomes unstable in those tracks. A control data area is also provided across plural tracks at either the outside or inside circumference area of the disk for recording the control data when computer data is recorded. However, this control data area is not always fully recorded with the control data, and the blank (unrecorded) regions thereof is recorded with dummy data, such as "FF" in hexadecimal code.
The tracking control band is usually several kilohertzes wide, and tracking control is disrupted if signal bands having a strong correlation are present in this control band. For example, if the record carrier rotates at 1800 rpm, tracking control will be disrupted by a strong inter-track signal correlation only several millimeters long at a 35 mm radial position.
Although the uniform data pattern and the narrow pitched tracks contributing to the strong correlation and crosstalk between neighboring tracks are described with respect to the tracking control, this goes for the focus control and the data reproduction operations.